Stellar masses, metallicity gradients, and suppressed star formation revealed in a new sample of absorption selected galaxies

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
e-mail: henrikrhodin@dark-cosmology.dk
² European Southern Observatory, Karl-Schwarzchildstrasse 2, 85748 Garching bei München, Germany
³ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
⁴ The Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark

Received: 9 March 2018
Accepted: 2 July 2018

Abstract

Context. Absorbing galaxies are selected via the detection of characteristic absorption lines which their gas-rich media imprint in the spectra of distant light-beacons. The proximity of the typically faint foreground absorbing galaxies to bright background sources makes it challenging to robustly identify these in emission, and hence to characterise their relation to the general galaxy population.

Aims. We search for emission to confirm and characterise ten galaxies hosting damped, metal-rich quasar absorbers at redshift z < 1.

Methods. We identified the absorbing galaxies by matching spectroscopic absorption -and emission redshifts and from projected separations. Combining emission-line diagnostics with existing absorption spectroscopy and photometry of quasar-fields hosting metal-rich, damped absorbers, we compare our new detections with reference samples and place them on scaling relations.

Results. We spectroscopically confirm seven galaxies harbouring damped absorbers (a 70% success-rate). Our results conform to the emerging picture that neutral gas on scales of tens of kpc in galaxies is what causes the characteristic H I absorption. Our key results are: (I) Absorbing galaxies with log₁₀[M_⋆,(M_⊙)] ≳ 10 have star formation rates that are lower than predicted for the main sequence of star formation. (II) The distribution of impact parameter with H I column density and with absorption-metallicity for absorbing galaxies at z ∼ 2–3 extends to z ∼ 0.7 and to lower H I column densities. (III) A robust mean metallicity gradient of ⟨Γ⟩ = −0.022 ± 0.001 dex kpc⁻¹. (IV) By correcting absorption metallicities for ⟨Γ⟩ and imposing a truncation-radius at 12 kpc, absorbing galaxies fall on top of predicted mass-metallicity relations, with a statistically significant decrease in scatter.